Outgoing FBI Director Robert Mueller is concerned about growing domestic terror threats, and says he is particularly worried about Americans traveling to Syria and coming back with intentions “to undertake an attack upon the homeland.”

Speaking to reporters on Thursday before he departs
the FBI next month, Mueller said terrorist threats are now
originating in countries such as Tunisia, Libya, Mali, Algeria,
Syria and Egypt. Syria, in particular, had been recruiting
fighters from around the world, including Americans, he noted.

“Every one of these countries now has cadres of individuals
who you would put in the category of extremists, violent
extremists that will present threats down the road, not only
threats to us here in the United States, but also threats to
Americans overseas,” Mueller said.

The United Nations estimates that at least 100,000 people have
died since the Syrian uprising began in 2011. More Westerners
have traveled to Syria to participate in the conflict than the
number of those who fought in Iraq, Afghanistan, Somalia or
Yemen. Matthew G. Olsen, the director of the National
Counterterrorism Center, last month called Syria “the predominant
jihadist battlefield in the world.”

And Mueller, who served as FBI director for 12 years, is
concerned that Syria could breed a radicalized class of American
terrorists that may conduct further attacks in the US.

“[When] you have individuals traveling to those venues, you
are concerned [first] about the associations they will make, and
secondly about the expertise they will develop and whether or not
they will utilize those associations, utilize that expertise, to
undertake an attack upon the homeland,” Mueller told ABC
News’ Pierre Thomas in an exclusive interview.

Mueller said FBI officials are concerned about and monitoring the
flow of American fighters into Syria.

But Mueller is not just concerned about American fighters who
might be radicalized in Syria; the outgoing FBI director said he
is also troubled about the possibility that terrorists might use
biological weapons of mass destruction, or that domestic attacks
could be conducted by American citizens that he described as
“lone wolf” suspects.

He pointed to the rising danger of homegrown terrorism, such as
the attacks at Fort Hood and the Boston Marathon. Although Boston
bombing suspect Tamerlan Tsarnaev came “to our attention”
before the attack occurred, Mueller said the FBI could do nothing
more beyond conducting its investigation. Unless there is stark
evidence that a terrorism suspect has plans to conduct an attack,
federal authorities are not able to take any action against a
suspect.

“In the future, we will have a case where the person has been
on our radar screen, but we do not have the evidence and there is
no outward evidentiary capability to bring the person into the
criminal justice system,” Mueller warned, expressing fear
that further domestic terrorist attacks might occur in the
future. “That is going to happen.”

Mueller’s term as FBI director expires on Sept. 4.
He will be succeeded by former US Deputy Attorney General
James
Comey.